The present invention relates to a novel catalyst for the conversion of sulfurous anhydride to sulfuric anhydride, and more particularly, it provides a novel catalyst comprising iron, copper, and at least one alkali metal chosen from the group of sodium and potassium, the catalyst being capable of enduring severe thermal shock, as well as processes for producing and using the catalyst.
In the preparation of sulfuric acid by the contact process, the conversion of sulfur dioxide to sulfur trioxide is usually effected in a number of steps in the presence of a catalyst containing vanadium at a temperature of from 400.degree. C. to 600.degree. C. at a pressure equal to or below 15 bars.
Laws for the control of pollution ordain that the residual content of sulfur dioxide in the exhaust gases be less and less, and this requires that the conversion yields of SO.sub.2 to SO.sub.3 be very high, on the order of 0.007 to 0.998. Such efficiencies are actually obtained at atmospheric pressure with intermediate absorption of the SO.sub.3 formed, but are more easily obtained with a pressure of 5 to 15 bars with or without intermediate absorption of the SO.sub.3 produced.
The vanadium catalysts are extremely active, and so none but the smallest quantities are utilized. It follows that when the operation is carried out under pressure the temperature gradient of the catalyst bed is very high. The control of the thermal stability of the catalyst mass is for this reason very difficult. There is thus a very considerable risk of catalyst destruction if a temperature of about 620.degree. C. is exceeded.
Accordingly, there is a considerable commercial need to devise a catalyst for the conversion of SO.sub.2 to SO.sub.3 which simultaneously has good activity at temperature on the order of 600.degree. C. and will withstand the stress of temperatures up to 850.degree. C. without degradation.
The inverse reaction for causing SO.sub.3 to revert to SO.sub.2, which occurs for instance in the recovery of residual acids and the conversion of available energy starting from solar or nuclear reactors, is very slow in the absence of a catalyst. There thus also exists a commercial requirement for the preparation of a catalyst capable of use at 600.degree.- 850.degree. C. for causing the reversion of SO.sub.3 to SO.sub.2.